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We examine the radial distributions of stellar populations in the globularcluster (GC) M15, using HST/WFC3 photometry of red giants in thenitrogen-sensitive F343N-F555W color. Surprisingly, we find that giants with"primordial" composition (i.e., N abundances similar to those in field stars)are the most centrally concentrated within the WFC3 field. We then combine ourWFC3 data with SDSS u, g photometry and find that the trend reverses for radii>1' (3 pc) where the ratio of primordial to N-enhanced giants increasesoutwards, as already found by Lardo et al. The ratio of primordial to enrichedstars thus has a U-shaped dependency on radius with a minimum near thehalf-light radius. N-body simulations show that mass segregation might producea trend resembling the observed one, but only if the N-enhanced giants are~0.25 Mo less massive than the primordial giants, which requires extreme Heenhancement (Y~0.40). However, such a large difference in Y is incompatiblewith the negligible optical color differences between primordial and enrichedgiants which suggest Delta Y < 0.03 and thus a difference in turn-off mass ofDelta M < 0.04 Mo between the different populations. The radial trends in M15are thus unlikely to be of dynamical origin and presumably reflect initialconditions, a result that challenges all current GC formation scenarios. Wenote that population gradients in the central regions of GCs remain poorlyinvestigated and may show a more diverse behavior than hitherto thought.

Radial distributions of sub-populations in the globular cluster M15: a more centrally concentrated primordial population